WSP11633

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 3/1/1994
• Title: Comments regarding the Draft Biological Opinion on Operations of Glen Canyon Dam
• Water Supply Pro - Doc Type: Biological Opinion

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GRAND CANYON NATIVE FISH MANAGEMENT

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The descending limb of spring peak flow discharges should progress in monthly steps, but
be concentrated in several successive days of each month to alIow relatively long periods of
stable flows in interim periods. Return to base flows should be accomplished by September,
. and these flows optimally shouIdbe maintained through February. The leveI of base flows
. . should be betweenS,ooo cfs and 10,000 cfs to provide Iarge numbers of mainstem rearing
habitats and to allow sufficient water to meet spring-summer needs.

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The proposed scenario for declining flows will afford young native fishes in Iower tributary
reaches the greatest possibIe opportunity to remain in these areas, yet retain reIatively
extended periods of stability in nearshore main stem rearing habitats for individuals that enter
this more hostile environment. The major advantages accrued by young native fishes from
their persistence in these relatively stabIe habitats are increased growth and survivorship due
to warmer water temperatures, lower current velocities, and increased availability of food
resources. These advantages will be multiplied to the extent that competitory and predatory
non-native fishes have been successfully displaced.

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Recommended Hydrology in the Presence of. Temperature Modijicatioti--Further
optimization of Glen Canyon Dam operations to benefit native fishes should incIude serious
consideration of a multi-IeveI intake structure (MUS) to control water temperature. At a
minimum; temperature modification is the only way to alleviate the known restriction by cold
water temperatures to successful mainstem reproduction by native species. We propose a .
gradual increase from hypolimnetic (deep water) release temperatures beginning in spring
when epilimnial (near surface water) temperatures exceed those of the hypolimnion in Lake
Powell. Epilimnial releases shouId be scheduIed to achieve a high of approximately IS.C
from 1une through September or October, followed by a rapid return to hypolimnetic release
temperatures in November through Febniary. This. pattern wouId~ provide..sufficient.
temperature elevation to allow for mainstem reproduction, and survival and accelerated
growth of young. We suggest altering the specific pattern of temperature modification
among years to mimic the variability inherent in the pre-dam ecosystem.

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Based on best available information" these actions would elevate -downstream summer
temperatures an-additional Z.C at theLittIe CoIorado River(LCR) (122km downstream) and
by nearly '.C at Diamond Creek (386 km downstream) above release temperatures due to
.downstream warming (Ferrari 1988). Thus, reIeases of IS.C in 1une and 1uIy should warm
to 19-20.C-optimum for spawning, incubation, and rearing--in the mainstem reach
beginning some 200 kIn below the dam. Attainment of these temperatures wouId provide
an oppOrtunity for successfuI reproduction by aggregations of humpback chub collected in
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that. reach in recent years, while minimizing disruption of reproduction by individuals that
presently reproduce in the LCR. Since the minimum temperature of successful incubation
and hatching is near IS.C, conditions ~ wouId allow some successful reproduction would

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